1. Field of the Invention
The present invention relates generally to a control apparatus for controlling a four-wheel-drive automotive vehicle, and more particularly to techniques for minimizing the operation of a rear-wheel driving electric motor so as to reduce a temperature rise of this electric motor, by controlling the ratio of distribution of the vehicle drive force to the front and rear drive wheels according to a desired vehicle drive force, particular while the vehicle is being started.
The present invention also relates to a control apparatus for controlling an automotive vehicle wherein one of a front and a rear drive wheels is driven by a first drive power source, while the other of the front and rear drive wheels is driven by a second vehicle drive power source.
2. Discussion of Related Art
There is known a control apparatus for controlling a four-wheel-drive automotive vehicle wherein the front wheels are driven by a first drive power source in the form of an engine while the rear wheels are driven by a second drive power source in the form of an electric motor. This control apparatus is arranged to increase the output torque of the electric motor with respect to the output torque of the engine, according to the operating amount of an accelerator pedal or the opening angle of a throttle valve. An example of such a control apparatus is disclosed in JP-A-63-188528.
In the known control apparatus for the four-wheel-drive vehicle described above, the output of the electric motor is controlled on the basis of the operating amount of the accelerator pedal and the vehicle running speed such that the output of the electric motor is increased with an increase in the operating amount of the accelerator pedal. However, the known control apparatus is not satisfactory and is required to be further improved, regarding the four-wheel driving mode in which the output of the engine for driving the front wheels and the output of the electric motor for driving the rear wheels are controlled depending upon the running condition of the vehicle. Where the operator""s desired vehicle drive force is provided by the sum of the front wheel drive force and the rear wheel drive force in the four-wheel driving mode, for instance, the known control apparatus suffers from an inadequate control of the ratio of distribution of the vehicle drive force to the front and rear wheels, due to a change in the static condition and dynamic or running condition of the vehicle and a change in the road surface condition.
The known control apparatus indicated above has another drawback. That is, a permissible range of the output torque of the electric motor is not set depending upon the running condition of the vehicle. Accordingly, the electric motor may suffer from overheating, with a result of limiting the vehicle running condition in which the electric motor is operable, leading to deteriorated drivability of the vehicle. Namely, the output torque to be generated by the electric motor should be limited depending upon its operating temperature, in order to prevent overheating and other damages of the electric motor.
The prevent invention was made in view of the background prior art discussed above. It is a first object of the present invention to provide a control apparatus for controlling a four-wheel-drive automotive vehicle, which permits the vehicle to be driven with the operator""s desired drive force, with an optimum ratio of distribution of the vehicle drive force to the front and rear wheels, irrespective of changes in the static and dynamic condition of the vehicle and the road surface condition.
A second object of the present invention is to provide a control apparatus for controlling an automotive vehicle having two drive power sources for driving respective front and rear drive wheels, which control apparatus permits increased drivability of the vehicle, by reducing a limitation of the vehicle running conditions in which the second drive power source in the form of an electric motor is operated.
The first object may be achieved according to a first aspect of the present invention, which provides a vehicle control apparatus for controlling a four-wheel-drive automotive vehicle of a type which comprises a first drive power source for driving one of a pair of front wheels and a pair of rear wheels, and a second drive power source for driving the other of the pairs of front and rear wheels, characterized in that the vehicle control apparatus is operable to control a front drive force for driving the pair of front wheels and a rear drive force for driving the pair of rear wheels, depending upon a static and a dynamic state of the vehicle such that a sum of the front drive force and the rear drive force is equal to the obtained operator""s desired value of a vehicle drive force for driving said automotive vehicle, which operator""s desired value is obtained on the basis of an amount of operation of a manually operated vehicle accelerating member and a running speed of the vehicle.
In the vehicle control apparatus constructed according to the first aspect of this invention constructed as described above, the front and rear drive forces for driving the respective pairs of front and rear wheels are controlled on the basis of the static and dynamic states of the vehicle such that a sum of the front and rear drive forces is equal to the operator""s desired value of the vehicle drive force which is obtained on the basis of the amount of operation of the manually operated vehicle accelerating member and the running speed of the vehicle. This vehicle control apparatus permits the vehicle to be driven in the four-wheel driving mode such that the ratio of the front and rear drive forces with respect to each other suitably reflects the static and dynamic states of the vehicle, which includes the condition of the road surface on which the vehicle lies.
Preferably, the operator""s desired value of the vehicle drive force is calculated on the basis of the operating amount of an accelerator pedal and the vehicle running speed and according to a predetermined relationship between the operator""s desired value and the operating amount of the accelerator pedal and the vehicle running speed, and the front and rear drive forces for driving the front and rear wheels are controlled on the basis of the static state of the vehicle such as a load distribution ratio of the front and rear wheels, the dynamic state of the vehicle such as a difference between the rotating speeds of the front and rear wheels and a longitudinal acceleration value of the vehicle, and the road surface condition such as the friction coefficient and gradient of the road surface, such that the sum of the front and rear drive forces is equal to the calculated operator""s desired value of the vehicle drive force.
According to one preferred form of the vehicle control apparatus according to the first aspect of the invention, the first drive power source consists of a plurality of drive power sources, preferably, a plurality of drive power sources of different types. In this form of the control apparatus, at least one of the two or more drive power sources of the first drive power source can be operated in an operating condition in which the efficiency is high enough to assure a high degree of fuel economy of the vehicle.
Preferably, the second drive power source consists of at least one motor/generator which selectively function as an electric motor and an electric motor. This second drive power source is desirably used to drive the pair of rear wheels.
According to another preferred form of the vehicle control apparatus according to the first aspect of this invention, a ratio of the front drive force and the rear drive force with respect to each other is determined on the basis of the operator""s desired value of the vehicle drive force. For instance, the ratio is changed when the operator""s desired value of the vehicle drive force has been reduced below a predetermined threshold. This arrangement reduces the rear-wheel drive force, when the operator""s desired vehicle drive force is small to such an extend that there is not a risk of slipping of the drive wheels. This arrangement is effective to prevent an unnecessary consumption of an electric power and a temperature rise of the electric motor used to drive the rear wheels.
According to a further preferred form of the vehicle control apparatus, the static state of the vehicle includes a starting state of the vehicle, and the ratio of the front drive force and the rear drive force in the starting state of the vehicle is controlled on the basis of the operator""s desired value of the vehicle drive force. This arrangement assures an adequate control of the distribution of the front and rear drive forces depending upon the operator""s desired vehicle drive force, when the four-wheel-drive vehicle is started.
According to a still further preferred form of the vehicle control apparatus, the static state of the vehicle includes a starting state of the vehicle, and the ratio of the front drive force and the rear drive force in the starting state of the vehicle is controlled such that one of the front and rear drive forces which is used for driving one of the pairs of front and rear wheels which is driven by one of the first and second drive power sources whose performance is more adversely influenced by a rise of its operating temperature is smaller when the operator""s desired value of the vehicle drive force is smaller than a predetermined threshold than when the operator""s desired value is not smaller than the predetermined threshold. This arrangement is effective to reduce the thermal load of one of the first and second drive power sources that is more likely to be adversely influenced by a rise of the operating temperature, whereby the vehicle can be kept driven in the four-wheel driving mode for a relatively long time.
According to a yet further preferred form of the vehicle control apparatus, the static state of the vehicle includes a starting state of the vehicle, and the ratio of the front drive force and the rear drive force in the starting state of the vehicle is controlled such the rear drive force for driving the rear wheels driven by the second drive power source is smaller when the operator""s desired value of the vehicle drive force is smaller than a predetermined threshold than when the operator""s desired value is not smaller than the predetermined threshold. This arrangement reduces the rear drive force to be produced by the second drive power source, resulting in a reduction in the operating temperature of the second drive power source, so that the second drive power source can be operated in a wider range of operating condition.
The above-indicated predetermined threshold used for controlling the ratio of the front and rear drive forces with respect to each other is determined by the maximum vehicle drive force below which the drive wheels are not likely to slip on a road surface having a predetermined low value of friction coefficient. According to this arrangement, the ratio of the rear drive force is made smaller to reduce the output of the second drive power source, namely, the output of the rear-wheel drive electric motor, so as to prevent overheating of this electric motor, when the operator""s desired vehicle drive force is smaller than the threshold, that is, does not cause the front and rear drive wheels to slip on the road surface.
The object indicated above may also be achieved according to a second aspect of this invention, which provides a vehicle control apparatus for controlling a four-wheel-drive automotive vehicle of a type which comprises a first drive power source for driving one of a pair of front wheels and a pair of rear wheels, and a second drive power source for driving the other of the pair of front and rear wheels, the vehicle control apparatus being operable to drive the automotive vehicle in a four-wheel driving mode in which the pairs of front and rear wheels are driven by the first and second drive power sources, when the automotive vehicle is placed in any one of a starting state in which the vehicle is being started, an accelerating state in which the vehicle is being accelerated, and a low-xcexc running state in which the vehicle is running on a road surface whose friction coefficient is lower than a predetermined threshold, the vehicle control apparatus being operable to drive the automotive vehicle in a two-wheel driving state in which one of the pairs of front and rear wheels are driven, when the vehicle is not placed in any one of the starting state, accelerating state and the low-xcexc running state.
In the vehicle control apparatus constructed according to a second aspect of this invention described above, the automotive vehicle is driven in the four-wheel driving mode with both the front wheels and the rear wheels, when the vehicle is placed in one of the starting state, accelerating state and low-xcexc running state. Accordingly, the four-wheel driving mode ro the two-wheel running mode is selected depending upon the vvehicle state, so that unnecessary vehicle running in the four-wheel driving mode is avoided, whereby the overheating of the second drive power source driven in the four-wheel driving mode can be prevented.
According to one preferred form of the second aspect of the invention, the vehicle control apparatus is operable to drive the automotive vehicle in the four-wheel driving mode when a load acting on the vehicle is smaller than a predetermined threshold, namely, when the vehicle is in deceleration or in a coasting run without an operation of a brake operating member. This vehicle control apparatus permits the vehicle driving in the four-wheel driving mode under a relatively small load.
The first and second drive power sources may include at least one electric motor, or a drive power source capable of generating an electric energy, namely, a motor/generator which selectively functions as an electric motor and an electric generator. In this case, the electric motor (motor/generator) is operated such that the engine is operable in an operating condition in which the efficiency of the engine is sufficiently high.
The vehicle control apparatus according a preferred form of the second aspect of the invention is operable when the vehicle is in the starting state, to drive the vehicle with only the electric motor of the first or second drive power source, or with a drive power source of the first or second drive power source, which is capable of generating an electric energy. In this case, the vehicle can be started without an operation of the engine, leading to an improved fuel economy of the engine.
The vehicle control apparatus according to another preferred form of the second aspect of the invention is operable to operate at least one electric motor as an electric generator so as to provide a regenerative braking torque, when the vehicle is being braked or in a coasting run. This arrangement assures improved energy efficiency and fuel economy of the vehicle.
The vehicle control apparatus according to a further preferred form of the second aspect of the invention is operable to drive the automotive vehicle with only the engine of the first drive power source, or with both the engine and the drive power source or electric motor capable of generating an electric energy, when a load acting on the vehicle is larger than a predetermined threshold. This arrangement permits the vehicle to be driven in the four-wheel driving mode with a sufficiently large drive force.
The object indicated above may also be achieved according to a third aspect of this invention, which provide a vehicle control apparatus for controlling an automotive vehicle of a type wherein front wheels and rear wheels can be driven by a drive power source device, the vehicle control apparatus being operable to control a front drive force for driving the front wheels and a rear drive force for driving the rear wheels, depending upon a state of the automotive vehicle, such that a sum of the front drive force and the rear drive force is equal to an operator""s desired value of a vehicle drive force for driving the vehicle, which operator""s desired value is obtained on the basis of an amount of operation of a manually operated vehicle accelerating member and a running speed of the vehicle.
In the vehicle control apparatus according to the third aspect of the invention, the front drive force and the rear drive force are controlled depending upon the vehicle state, such that the sum of the front and rear drive forces is equal to the operator""s desired vehicle drive force obtained on the basis of the operating amount of the manually operated vehicle accelerating member and the vehicle running state. This vehicle control apparatus permits the vehicle to be driven in the four-wheel driving mode with the operator""s desired vehicle drive force, such that the front and rear drive forces suitably reflect the vehicle state.
Preferably, the front and rear wheels are operatively connected to a common drive power source device, and the ratio of the front drive force and the rear drive force with respect to each other is controlled by a drive force distributing clutch. This arrangement eliminates a need of providing a plurality of drive power sources at respective different locations on the vehicle.
In a further preferred form of the vehicle control apparatus according to the first or second aspect of this invention described above, the automotive vehicle has traction control means for reducing a drive force of the one of the pairs of front and rear wheels, so that a slip ratio of the one pair of wheels driven by the first drive power source is held within a predetermined optimum range, the vehicle control apparatus comprising (a) torque distribution feedback control means for controlling a front-rear torque distribution ratio which is a ratio of a front-wheel drive torque for driving the front wheels and a rear-wheel drive torque for driving the rear wheels with respect to each other, such that an actual state of slipping of the one pair of wheels with respect to that of the other pair of wheels coincides with a desired state of slipping, and (b) feedback control changing means operable when the traction control means is in operation, for commanding the torque distribution feedback control means to change the front-rear torque distribution ratio to a value different from that used when the traction control means is not in operation.
In the above arrangement, the front-rear torque distribution ratio is controlled by the feedback-controlled by the torque distribution feedback control means such that the actual slipping state of the front and rear wheels with respect to each other coincides with the desired value, so that the total drive torque is adequately distributed to the front and rear wheels. Further, the feedback control changing means commands the torque distribution feedback control means to control the front-rear torque distribution ratio to different values depending upon whether the traction control means is in operation or not. Namely, even where the drive torque of the drive wheels driven by the first drive power source is reduced to reduce the slipping tendency of those drive wheels as a result of the traction control by the traction control means, for instance, the drive torque of the other drive wheels driven by the second drive power source is increased to maintain substantially the same total vehicle drive force or torque, so that the vehicle can be driven in the four-wheel driving mode with high drivability.
Preferably, the vehicle control apparatus according to the above preferred arrangement further includes second-drive-source control means for controlling the second drive power source on the basis of the front-rear torque distribution ratio determined by the torque distribution feedback control means. In this instance, the actual front-rear torque distribution is controlled by an operation of the second drive power source so that the actual slip state of the front and rear wheels is made equal to the desired value.
The feedback control changing means indicated above is preferably adapted to command the torque distribution feedback control means, during an operation of the traction control means, to change at least one of a) a control error of the slip state of the wheels to be controlled by feedback control by the torque distribution feedback control means, b) a desired value of the slip state, and c) an actual value of the slip state, the control error being a difference between the desired and actual values, such that the ratio of the above-indicated other pair of wheels driven by the second drive power source is increased. In this arrangement wherein at least one of the control error of the slip state and the desired and actual values of the slip state which determine the control error is changed so as to increase the ratio of the drive torque of the wheels driven by the second drive power source, the vehicle can be driven with high drivability, with the second drive power source being operated to drive the corresponding wheels with the increased drive torque even while the other drive wheels are under the traction control by the traction control means.
The feedback control changing means is preferably adapted to command the torque distribution feedback control means to change a feedback gain included in a feedback control equation used for calculating the front-rear torque distribution ratio, such that the changed feedback gain increase the ratio of the drive torque of the wheels driven by the second drive power source. This arrangement to change the feedback gain so as to increase the ratio of the drive torque of the wheels driven by the second drive power source permits the vehicle to be driven with high drivability with the second drive power source being operated to drive the corresponding wheels with an increased drive torque when the traction control means is in operation.
The feedback control changing means may be adapted to change, during an operation of the traction control means, the front-rear torque distribution ratio as calculated by the torque distribution feedback control means according to a control equation, such that the changed front-rear torque distribution ratio increases the ratio of the drive torque of the wheels driven by the second drive power source. In this arrangement, too, the second drive power source is operated to drive the corresponding wheels with an increased drive torque even while the traction control means is in operation.
Preferably, the traction control means is adapted to reduce the output of the first drive power source and/or the drive force of the wheels driven by the first drive power source, when the vehicle is started on a snow-covered or frozen road surface or any other road surface whose friction coefficient is lower than a predetermined lower limit. In this case, the front-rear torque distribution ratio is changed by the torque distribution feedback control means where the traction control is effected to reduce the output of the first drive power source and/or the drive force of the drive wheels driven by the first drive power source.
In a further preferred form of the vehicle control apparatus according to the first or second aspect of the invention, the first drive power source includes a first electric motor for driving the pair of front wheels while the second drive power source includes a second electric motor for driving the pair of rear wheels, the vehicle control apparatus being operable to control the first and second electric motors according to a predetermined relationship between thermal ratings of the first and second electric motors. This arrangement permits the vehicle to be driven with a high degree of running stability, while taking into account of the balance of the front-wheel drive force and the rear-wheel drive force.
Preferably, the thermal rating of the first electric motor is made higher than that of the second electric motor, so that the output of the second electric motor having the lower thermal rating is restricted or limited before that of the first electric motor is restricted or reduced. The restriction of the output of the second electric motor operated to drive the rear wheels assures a comparatively high degree of running stability of the vehicle.
In a still further preferred form of the vehicle control apparatus according to the first or second aspect of this invention, the first drive power source includes a first electric motor for driving the pair of front wheels while the second drive power source includes a second electric motor for driving the pair of rear wheels, and the first drive power source includes a first electric motor for driving the pair of front wheels while the second drive power source includes a second electric motor for driving the pair of rear wheels, the vehicle control apparatus including first-motor output increasing means operable when an output of the second electric motor is limited, to increase an output of the first electric motor. In the present arrangement wherein the output of the first electric motor is increased when the output of the second electric motor is limited, the vehicle can be driven with a comparatively high degree of stability, without a change in the total vehicle drive force. Where each of the first and second electric motor is a motor/generator capable of providing a regenerative braking torque, the vehicle can be braked with a comparatively high degree of stability, without a change in the total regenerative vehicle braking torque.
In the above form of the invention, too, the thermal rating of the first electric motor is preferably made higher than that of the second electric motor, so that the output of the second electric motor having the lower thermal rating is restricted or limited before that of the first electric motor. The restriction of the output of the second electric motor operated to drive the rear wheels assures a comparatively high degree of running stability of the vehicle.
In another preferred form of the vehicle control apparatus according to the first or second aspect of the present invention, the first drive power source includes a first electric motor for driving the pair of front wheels while the second drive power source includes a second electric motor for driving the pair of rear wheels, and the vehicle control apparatus includes second-motor output reducing means operable when an output of the first electric motor is limited, to reduce an output of the second electric motor so that a ratio of a front drive force for driving the front wheels and a rear drive force for driving the rear wheels with respect to each other coincides with a desired value. This arrangement to reduce the output of the second electric motor upon reduction of the output of the first electric motor permits the actual front-rear distribution ratio of the vehicle drive force or braking force to be maintained at the desired value, assuring a high degree of running stability of the vehicle. Namely, upon reduction of the output of the first electric motor, the output of the second electric motor is reduced so that the desired rear-wheel drive force ratio is maintained, or so that the actual front-wheel drive force is larger than the rear-wheel drive force by a desired amount. Where each of the first and second electric motors is a motor/generator, the regenerative braking force generated by the second electric motor is reduced when the regenerative braking force generated by the first electric motor is limited. Thus, the vehicle can be driven with high stability, without a change in the front-rear drive force distribution ratio.
Preferably, the thermal rating of the first electric motor is made higher than that of the second electric motor, so that the output of the second electric motor having the lower thermal rating is restricted or limited before that of the first electric motor is restricted or reduced. The restriction of the output of the second electric motor operated to drive the rear wheels assures a comparatively high degree of running stability of the vehicle.
The vehicle control apparatus according to a further preferred form of the first or second aspect of this invention is adapted to apply an assisting drive force to the automotive vehicle upon starting of the automotive vehicle on an uphill road surface, such that the assisting drive force corresponds to a gradient of the uphill road surface, the vehicle control apparatus including means for inhibiting the application of the assisting drive force to the automotive vehicle if a brake operating member for applying a brake to the automotive vehicle has been kept in its non-operated position for more than a predetermined time while the vehicle is stationary. A fact that the vehicle has been kept in its non-operated position for a relatively long time indicates that the vehicle operator does not has an intention of starting the vehicle. Since an assisting drive force is not applied to the vehicle in this case, the vehicle is permitted to be moved down in the reverse direction on the uphill road surface, so that the vehicle operator can recognize or perceive the gradient of the uphill road surface.
The vehicle control apparatus according to a further preferred form of the first or second aspect of this invention is adapted to apply an assisting drive force to the automotive vehicle on an uphill road surface, such that the assisting drive force corresponds to a gradient of the uphill road surface, the vehicle control apparatus including means for generating the assisting drive force such that the assisting drive force rapidly increases to a desired value for an initial period of the application of the assisting drive force, and slowly decreases from the desired value to zero for a terminal period of the application of the assisting drive force. According to this arrangement, the vehicle can be smoothly started on the uphill road surface, with the assisting drive force being rapidly increased to the desired value, so that the vehicle is suitably prevented from being moved down in the reverse direction on the uphill road surface upon starting of the vehicle. Further, the assisting drive force is slowly reduced after the vehicle has been started on the uphill road surface, so that the assisting drive force is removed without giving a discomfort to the vehicle operator.
In a further preferred form of the vehicle control apparatus according to the first or second aspect of the invention, the front drive force for driving the pair of front wheels and the rear drive force for driving the pair of rear wheels are controlled, during starting of the vehicle, on the basis of a gradient of a road surface on which the vehicle is started. In this arrangement, the front and rear drive force the sum of which is determined by the operator""s desired vehicle drive force are controlled on the basis of the gradient of the road surface while the vehicle is being started.
The vehicle control apparatus according to the above preferred form of the invention is desirably adapted to determine the vehicle drive force on the basis of the gradient of the road surface such that a speed at which the vehicle is moved down in the reverse direction on an uphill road surface is lower than a predetermined threshold, as long as the road surface gradient is held within a predetermined range. In other words, the vehicle drive force is not increased after the road surface gradient exceeds the upper limit of the predetermined range, so that the vehicle operator can perceive the relatively large road surface gradient with high accuracy, since the vehicle is more or less moved down on the road surface in the reverse direction if the road surface gradient is excessively large.
The above-indicated predetermined threshold or upper limit of the speed of the backward movement of the vehicle on the uphill road surface is preferably several km/h, for instance, about 1-3 km/h. In this case, the vehicle is prevented from moving down on the uphill road surface at a high speed exceeding about 3 km/h, for example.
However, the vehicle drive force may be determined on the basis of the gradient of the road surface such that an acceleration value of the vehicle when the vehicle is moved down in the reverse direction on the uphill road surface is lower than a predetermined threshold, for instance, about 1.0 m/s2.
The application of the vehicle drive force corresponding to the road surface gradient may be terminated when the operator""s desired vehicle drive force has exceeded a predetermined upper limit. In this arrangement, the vehicle drive force is increased with an increase in the road surface gradient, to prevent or reduce the downward reverse movement of the vehicle on the uphill road surface, as long as the operator""s desired vehicle drive force is smaller than the upper limit.
The second object indicated above may be achieved according to a third aspect of this invention, which provides a vehicle control apparatus for controlling a four-wheel-drive automotive vehicle of a type which comprises a first drive power source for driving one of a pair of front wheels and a pair of rear wheels, and a second drive power source for driving the other of the pairs of front and rear wheels, the vehicle control apparatus comprising:
output-torque-range selecting means for selecting one of a plurality of output torque ranges in which the second drive power source is operated, on the basis of an operating state of the automotive vehicle; and second-power-source control means for operating the second drive power source such that an output torque of the second drive power source is held within the selected output torque range.
In the vehicle control apparatus constructed according to the third aspect of the present invention, the second drive power source is operated such that the output torque of the second drive power source is held within one of the output torque ranges which is selected by the output-torque-range selecting means on the basis of the operating state of the vehicle. This arrangement permits the automotive vehicle to be driven, with the second drive power source being operated to provide the required minimum drive force. Accordingly, there does not arise an overheating or a temperature rise of the second drive power source, which would limit the operation of the second drive power source. Thus, the present arrangement reduces the limitation in the operation of the second drive power source, making it possible to improve the drivability of the vehicle.
Preferably, the vehicle control apparatus according to the third aspect of the invention further comprises output-torque-range memory means for storing data representative of the plurality of output torque ranges.
In one preferred form of the vehicle control apparatus according to the third aspect of the invention, the plurality of output torque ranges include at least a first range of the output torque and a second range of the output torque whose upper limit is smaller than that of the first range. This arrangement permits the second drive power source to be operated in the second output torque range whose upper limit is comparatively small. Accordingly, an operation of the second drive power source for a long time to provide a comparatively large drive torque in the first range is avoided, thereby preventing an overheating or temperature rise of the second drive power source, so that the second drive power source can be maintained in an operable state. For instance, the two or more output torque ranges are defined in a two-dimensional coordinate system in which the operating speed of the second drive power source is taken along a first axis while the output torque of the second drive power source is taken along a second axis perpendicular to the first axis. The first output torque range has a larger maximum output torque value than that of the second output torque range. One of these first and second output torque ranges is selected depending upon the operating or running state of the vehicle, for driving the vehicle in the four-wheel drive mode with an operation of the second drive power source as well as an operation of the first drive power source, such that the output torque of the second drive power source is reduced as much as possible. The present arrangement prevents a continued operation of the second drive power source in the first output toque range in which the output torque is comparatively large, so that the second drive power source is kept operable.
In another preferred form of the vehicle control apparatus according to the third aspect of the invention, the second-power-source control means reduces the output torque of the second drive power source across a boundary of two adjacent ones of the plurality of output torque ranges, at a rate lower than a rate at which the second-power-source control means increases the output torque across the boundary. This arrangement prevents a rapid reduction of the drive force of the wheels driven by the second drive power source, assuring running stability of the vehicle. When the selected output torque range of the second drive power source is changed from the range whose maximum drive toque is relatively large to the range whose maximum drive torque is relatively small, the second-power-source control means reduces the output torque of the second drive power source at a rate lower than the rate at which the output torque is increased when the selected output torque range is changed from the range whose maximum drive torque is relatively small to the range whose maximum drive torque is relatively large. Accordingly, an abrupt reduction of the drive force of the wheels driven by the second drive power source is avoided, assuring a sufficiently high degree of running stability of the vehicle.
In a still further preferred form of the vehicle control apparatus according to the third aspect of the invention, the output-torque-range selecting means selects the above-indicated first range when the automotive vehicle is placed in any one of a starting state, a slipping state of its drive wheels and an understeering state, and selects the above-indicated second range when the automotive vehicle is placed in any other states. This arrangement permits the second drive power source to be operated to provide a sufficiently large drive torque when the vehicle is in a starting or understeering state or when the drive wheels are slipping on the road surface. For instance, the output-torque-range selecting means is adapted to select the first output torque range when the wheels driven by the first drive power source are slipping. The present form of the vehicle control apparatus is effective to permit smooth starting and acceleration of the vehicle, and prevent slipping of the wheels driven by the first drive power source and an understeering state of the vehicle.
In a yet further preferred form of the vehicle control apparatus according to the third aspect of this invention, the output-torque-range selecting means selects the above-indicated first range when a gradient of a road surface on which the automotive vehicle lies is larger than a predetermined threshold, and selects the above-indicated second range when the gradient is not larger than the predetermined threshold. This arrangement is effective to prevent or reduce the reverse downward movement of the vehicle on an uphill road surface, while maximizing the frequency of operation of the second drive power source in the second output torque range so as to provide the required minimum drive torque. Thus, the present arrangement assures improved efficiency of operation of the second drive power source and effectively prevents overheating of the second drive power source, particularly where the second drive power source is an electric motor.
In still another preferred form of the vehicle control apparatus according to the third aspect of this invention, the second-drive-source control means operates the second drive power source during starting of the automotive vehicle on an uphill road surface, such that the automotive vehicle is kept driven in a four-wheel driving mode with the pairs of front and rear wheels, until the running speed of the vehicle is raised to a higher value when the gradient of the uphill road surface is relatively large, than when the gradient is relatively small. This arrangement is effective to prevent or reduce the reverse downward movement of the vehicle on the uphill road surface when the vehicle is started on the uphill road surface.
Preferably, the vehicle control apparatus according to the third aspect of the invention further comprises (a) an ant-lock braking-pressure control means operable upon detecting of slipping of any one of the wheels on the basis of the peripheral speed of each wheel detected by a wheel speed sensor, for controlling a braking force to be applied to each slipping wheel such that a slip ratio of the slipping wheel is held within a predetermined range, and (b) vehicle-turning-stability control means operable during turning of the vehicle, for controlling a braking force or drive force of an appropriate one or ones of the wheels so as to prevent an understeering or oversteering state off the vehicle, such that a direction of running of the vehicle does not deviate from a nominal running path of the vehicle defined by a steering angle of a steering wheel of the vehicle, and wherein the second-power-source control means terminates or inhibits an operation of the second drive power source when said wheel speed sensor is defective, or when one of the anti-lock braking-pressure control means and the vehicle-turning-stability control means is in operation. This arrangement automatically changes the vehicle driving mode from the four-wheel driving mode to the two-wheel driving mode (in which the vehicle is driven with only the first drive power source), when the wheel speed sensor is defective or when the anti-lock braking-pressure control means or the vehicle-turning-stability control means is in operation. The present arrangement prevents a control interference between different controls including the control of the second drive power source by the second-power-source control means, the anti-lock braking-pressure control by the anti-lock braking-pressure control means and the vehicle-turning-stability control by the vehicle-turning-stability control means, so that the running stability and safety of the vehicle is improved.
Preferably, the vehicle control apparatus according to the third aspect of the invention further comprises low-temperature detecting means for detecting that an ambient temperature of the vehicle is lower than a predetermined lower limit below which a friction coefficient of a road surface on which the vehicle is running is expected to be lower than a predetermined lower limit, and wherein the second-power-source control means operates the second drive power source when the ambient temperature lower than the lower limit is detected by the low-temperature detecting means. In this arrangement, the second drive power source is automatically activated upon detection of the ambient temperature lower than the lower limit, so that the running safety of the vehicle is improved.
Preferably, the control apparatus according to the third aspect of this invention further comprises (a) vehicle-starting detecting means for determining whether the vehicle is in the process of being started, (b) wheel-slip detecting means for determining whether the wheels are slipping on a road surface, (c) understeering detecting means for determining, on the basis of a steering angle and a yaw rate of the vehicle, whether the vehicle is turning in an understeering state, (d) vehicle-turning detecting means for determining whether the vehicle is turning with the steering angle being larger than a predetermined threshold, (e) accelerator operation detecting means for determining whether a vehicle accelerating member is operated at a rate higher than a predetermined threshold, (f) high-load running detecting means for determining whether the vehicle is running under a relatively high load with the vehicle accelerating member being operated by more than a predetermined amount, and (g) vehicle-deceleration detecting means for determining whether the vehicle is in a decelerating state, and wherein the second-power-source control means determines that the vehicle is required to be driven in the four-wheel driving mode, and operates the second drive power source, when an affirmative decision is obtained by any one of the vehicle-starting detecting means, the wheel-slip detecting means, the understeering detecting means, the vehicle turning-detecting means, the accelerator operation detecting means, the high-load running detecting means and the vehicle-deceleration detecting means, the second-power-source control means determining that the vehicle is not required to be driven in the four-wheel driving mode when a negative decision is obtained by all of the above-indicated seven detecting means, and turning off the second drive power source a predetermined delay time after the moment of the determination that the vehicle is not required to be driven in the four-wheel driving mode. According to this arrangement, the second drive power source is automatically operated when the vehicle driving in the four-wheel driving mode is required, so that the vehicle can be driven with high stability. Further, the delay time provided after the determination that the four-wheel driving of the vehicle is not required and before the second drive power source is turned off is effective to prevent a on-off control hunting of the second drive power source.
Preferably, the vehicle control apparatus according to the above-arrangement further comprises at least one of (a) steering-angle sensor defect detecting means for detecting a defect of a steering angle sensor for detecting the steering angle of the steering wheel of the vehicle, and (b) a yaw-rate sensor defect detecting means for detecting a defect of a yaw rate sensor for detecting the yaw rate of the vehicle, and wherein the second-power-source control means does not operate the second drive power source if the defect of either one of the steering angle sensor and the yaw rate sensor is detected, even when the understeering detecting means has detected the understeering state of the vehicle. This arrangement prevents the vehicle driving in the four-wheel driving mode even if the understeering state of the vehicle is erroneously detected due to a defect of the steering angle sensor or yaw rate sensor.
In a further preferred form of the vehicle control apparatus according to the third aspect of this invention described above, the automotive vehicle has traction control means for reducing a drive force of the one of the pairs of front and rear wheels, so that a slip ratio of the one pair of wheels driven by the first drive power source is held within a predetermined optimum range, the vehicle control apparatus comprising (a) torque distribution feedback control means for controlling a front-rear torque distribution ratio which is a ratio of a front-wheel drive torque for driving the front wheels and a rear-wheel drive torque for driving the rear wheels with respect to each other, such that an actual state of slipping of the one pair of wheels with respect to that of the other pair of wheels coincides with a desired state of slipping, and (b) feedback control changing means operable when the traction control means is in operation, for commanding the torque distribution feedback control means to change the front-rear torque distribution ratio to a value different from that used when the traction control means is not in operation.
In the above arrangement, the front-rear torque distribution ratio is controlled by the feedback-controlled by the torque distribution feedback control means such that the actual slipping state of the front and rear wheels with respect to each other coincides with the desired value, so that the total drive torque is adequately distributed to the front and rear wheels. Further, the feedback control changing means commands the torque distribution feedback control means to control the front-rear torque distribution ratio to different values depending upon whether the traction control means is in operation or not. Namely, even where the drive torque of the drive wheels driven by the first drive power source is reduced to reduce the slipping tendency of those drive wheels as a result of the traction control by the traction control means, for instance, the drive torque of the other drive wheels driven by the second drive power source is increased to maintain substantially the same total vehicle drive force or torque, so that the vehicle can be driven in the four-wheel driving mode with high drivability.
Preferably, the vehicle control apparatus according to the above preferred arrangement further includes second-drive-source control means for controlling the second drive power source on the basis of the front-rear torque distribution ratio determined by the torque distribution feedback control means. In this instance, the actual front-rear torque distribution is controlled by an operation of the second drive power source so that the actual slip state of the front and rear wheels is made equal to the desired value.
The feedback control changing means indicated above is preferably adapted to command the torque distribution feedback control means, during an operation of the traction control means, to change at least one of a) a control error of the slip state of the wheels to be controlled by feedback control by the torque distribution feedback control means, b) a desired value of the slip state, and c) an actual value of the slip state, the control error being a difference between the desired and actual values, such that the ratio of the above-indicated other pair of wheels driven by the second drive power source is increased. In this arrangement wherein at least one of the control error of the slip state and the desired and actual values of the slip state which determine the control error is changed so as to increase the ratio of the drive torque of the wheels driven by the second drive power source, the vehicle can be driven with high drivability, with the second drive power source being operated to drive the corresponding wheels with the increased drive torque even while the other drive wheels are under the traction control by the traction control means.
The feedback control changing means is preferably adapted to command the torque distribution feedback control means to change a feedback gain included in a feedback control equation used for calculating the front-rear torque distribution ratio, such that the changed feedback gain increase the ratio of the drive torque of the wheels driven by the second drive power source. This arrangement to change the feedback gain so as to increase the ratio of the drive torque of the wheels driven by the second drive power source permits the vehicle to be driven with high drivability with the second drive power source being operated to drive the corresponding wheels with an increased drive torque when the traction control means is in operation.
The feedback control changing means may be adapted to change, during an operation of the traction control means, the front-rear torque distribution ratio as calculated by the torque distribution feedback control means according to a control equation, such that the changed front-rear torque distribution ratio increases the ratio of the drive torque of the wheels driven by the second drive power source. In this arrangement, too, the second drive power source is operated to drive the corresponding wheels with an increased drive torque even while the traction control means is in operation.
Preferably, the traction control means is adapted to reduce the output of the first drive power source and/or the drive force of the wheels driven by the first drive power source, when the vehicle is started on a snow-covered or frozen road surface or any other road surface whose friction coefficient is lower than a predetermined lower limit. In this case, the front-rear torque distribution ratio is changed by the torque distribution feedback control means where the traction control is effected to reduce the output of the first drive power source and/or the drive force of the drive wheels driven by the first drive power source.
In a further preferred form of the vehicle control apparatus according to the third aspect of the invention, the first drive power source includes a first electric motor for driving the pair of front wheels while the second drive power source includes a second electric motor for driving the pair of rear wheels, the vehicle control apparatus being operable to control the first and second electric motors according to a predetermined relationship between thermal ratings of the first and second electric motors. This arrangement permits the vehicle to be driven with a high degree of running stability, while taking into account of the balance of the front-wheel drive force and the rear-wheel drive force.
Preferably, the thermal rating of the first electric motor is made higher than that of the second electric motor, so that the output of the second electric motor having the lower thermal rating is restricted or limited before that of the first electric motor is restricted or reduced. The restriction of the output of the second electric motor operated to drive the rear wheels assures a comparatively high degree of running stability of the vehicle.
In a still further preferred form of the vehicle control apparatus according to the third aspect of this invention, the first drive power source includes a first electric motor for driving the pair of front wheels while the second drive power source includes a second electric motor for driving the pair of rear wheels, and the first drive power source includes a first electric motor for driving the pair of front wheels while the second drive power source includes a second electric motor for driving the pair of rear wheels, the vehicle control apparatus including first-motor output increasing means operable when an output of the second electric motor is limited, to increase an output of the first electric motor. In the present arrangement wherein the output of the first electric motor is increased when the output of the second electric motor is limited, the vehicle can be driven with a comparatively high degree of stability, without a change in the total vehicle drive force. Where each of the first and second electric motor is a motor/generator capable of providing a regenerative braking torque, the vehicle can be braked with a comparatively high degree of stability, without a change in the total regenerative vehicle braking torque.
In the above form of the invention, too, the thermal rating of the first electric motor is preferably made higher than that of the second electric motor, so that the output of the second electric motor having the lower thermal rating is restricted or limited before that of the first electric motor. The restriction of the output of the second electric motor operated to drive the rear wheels assures a comparatively high degree of running stability of the vehicle.
In another preferred form of the vehicle control apparatus according to the third aspect of the present invention, the first drive power source includes a first electric motor for driving the pair of front wheels while the second drive power source includes a second electric motor for driving the pair of rear wheels, and the vehicle control apparatus includes second-motor output reducing means operable when an output of the first electric motor is limited, to reduce an output of the second electric motor so that a ratio of a front drive force for driving the front wheels and a rear drive force for driving the rear wheels with respect to each other coincides with a desired value. This arrangement to reduce the output of the second electric motor upon reduction of the output of the first electric motor permits the actual front-rear distribution ratio of the vehicle drive force or braking force to be maintained at the desired value, assuring a high degree of running stability of the vehicle. Namely, upon reduction of the output of the first electric motor, the output of the second electric motor is reduced so that the desired rear-wheel drive force ratio is maintained, or so that the actual front-wheel drive force is larger than the rear-wheel drive force by a desired amount. Where each of the first and second electric motors is a motor/generator, the regenerative braking force generated by the second electric motor is reduced when the regenerative braking force generated by the first electric motor is limited. Thus, the vehicle can be driven with high stability, without a change in the front-rear drive force distribution ratio.
Preferably, the thermal rating of the first electric motor is made higher than that of the second electric motor, so that the output of the second electric motor having the lower thermal rating is restricted or limited before that of the first electric motor is restricted or reduced. The restriction of the output of the second electric motor operated to drive the rear wheels assures a comparatively high degree of running stability of the vehicle.
The vehicle control apparatus according to a further preferred form of the first or second aspect of this invention is adapted to apply an assisting drive force to the automotive vehicle upon starting of the automotive vehicle on an uphill road surface, such that the assisting drive force corresponds to a gradient of the uphill road surface, the vehicle control apparatus including means for inhibiting the application of the assisting drive force to the automotive vehicle if a brake operating member for applying a brake to the automotive vehicle has been kept in its non-operated position for more than a predetermined time while the vehicle is stationary. A fact that the vehicle has been kept in its non-operated position for a relatively long time indicates that the vehicle operator does not has an intention of starting the vehicle. Since an assisting drive force is not applied to the vehicle in this case, the vehicle is permitted to be moved down in the reverse direction on the uphill road surface, so that the vehicle operator can recognize or perceive the gradient of the uphill road surface.
The vehicle control apparatus according to a further preferred form of the first or second aspect of this invention is adapted to apply an assisting drive force to the automotive vehicle on an uphill road surface, such that the assisting drive force corresponds to a gradient of the uphill road surface, the vehicle control apparatus including means for generating the assisting drive force such that the assisting drive force rapidly increases to a desired value for an initial period of the application of the assisting drive force, and slowly decreases from the desired value to zero for a terminal period of the application of the assisting drive force. According to this arrangement, the vehicle can be smoothly started on the uphill road surface, with the assisting drive force being rapidly increased to the desired value, so that the vehicle is suitably prevented from being moved down in the reverse direction on the uphill road surface upon starting of the vehicle. Further, the assisting drive force is slowly reduced after the vehicle has been started on the uphill road surface, so that the assisting drive force is removed without giving a discomfort to the vehicle operator.
In a further preferred form of the vehicle control apparatus according to the first or second aspect of the invention, the front drive force for driving the pair of front wheels and the rear drive force for driving the pair of rear wheels are controlled, during starting of the vehicle, on the basis of a gradient of a road surface on which the vehicle is started. In this arrangement, the front and rear drive force the sum of which is determined by the operator""s desired vehicle drive force are controlled on the basis of the gradient of the road surface while the vehicle is being started.
The vehicle control apparatus according to the above preferred form of the invention is desirably adapted to determine the vehicle drive force on the basis of the gradient of the road surface such that a speed at which the vehicle is moved down in the reverse direction on an uphill road surface is lower than a predetermined threshold, as long as the road surface gradient is held within a predetermined range. In other words, the vehicle drive force is not increased after the road surface gradient exceeds the upper limit of the predetermined range, so that the vehicle operator can perceive the relatively large road surface gradient with high accuracy, since the vehicle is more or less moved down on the road surface in the reverse direction if the road surface gradient is excessively large.
The above-indicated predetermined threshold or upper limit of the speed of the backward movement of the vehicle on the uphill road surface is preferably several km/h, for instance, about 1-3 km/h. In this case, the vehicle is prevented from moving down on the uphill road surface at a high speed exceeding about 3 km/h, for example.
However, the vehicle drive force may be determined on the basis of the gradient of the road surface such that an acceleration value of the vehicle is moved down in the reverse direction on the uphill road surface is lower than a predetermined threshold, for instance, about 1.0 m/s2.
The uphill-starting control means 350 may be modified to apply an assisting drive force to the vehicle before starting of the vehicle on the uphill road surface, so that the acceleration value of the vehicle during downward movement in the reverse direction on the uphill road surface is held lower than a predetermined small value, for example, about 1.0 m/s2.
The application of the vehicle drive force corresponding to the road surface gradient may be terminated when the operator""s desired vehicle drive force has exceeded a predetermined upper limit. In this arrangement, the vehicle drive force is increased with an increase in the road surface gradient, to prevent or reduce the downward reverse movement of the vehicle on the uphill road surface, as long as the operator""s desired vehicle drive force is smaller than the upper limit.
The second object indicated above may be achieved according to a third aspect of this invention, which provides a vehicle control apparatus for controlling a four-wheel-drive automotive vehicle of a type which comprises a first drive power source for driving one of a pair of front wheels and a pair of rear wheels, and a second drive power source for driving the other of the pairs of front and rear wheels, the vehicle control apparatus comprising:
output-torque-range selecting means for selecting one of a plurality of output torque ranges in which the second drive power source is operated, on the basis of an operating state of the automotive vehicle; and
second-power-source control means for operating the second drive power source such that an output torque of the second drive power source is held within the selected output torque range.
In the vehicle control apparatus constructed according to the third aspect of the present invention, the second drive power source is operated such that the output torque of the second drive power source is held within one of the output torque ranges which is selected by the output-torque-range selecting means on the basis of the operating state of the vehicle. This arrangement permits the automotive vehicle to be driven, with the second drive power source being operated to provide the required minimum drive force. Accordingly, there does not arise an overheating or a temperature rise of the second drive power source, which would limit the operation of the second drive power source. Thus, the present arrangement reduces the limitation in the operation of the second drive power source, making it possible to improve the drivability of the vehicle.
Preferably, the vehicle control apparatus according to the third aspect of the invention further comprises output-torque-range memory means for storing data representative of the plurality of output torque ranges.
In one preferred form of the vehicle control apparatus according to the third aspect of the invention, the plurality of output torque ranges include at least a first range of the output torque and a second range of the output torque whose upper limit is smaller than that of the first range. This arrangement permits the second drive power source to be operated in the second output torque range whose upper limit is comparatively small. Accordingly, an operation of the second drive power source for a long time to provide a comparatively large drive torque in the first range is avoided, thereby preventing an overheating or temperature rise of the second drive power source, so that the second drive power source can be maintained in an operable state. For instance, the two or more output torque ranges are defined in a two-dimensional coordinate system in which the operating speed of the second drive power source is taken along a first axis while the output torque of the second drive power source is taken along a second axis perpendicular to the first axis. The first output torque range has a larger maximum output torque value than that of the second output torque range. One of these first and second output torque ranges is selected depending upon the operating or running state of the vehicle, for driving the vehicle in the four-wheel drive mode with an operation of the second drive power source as well as an operation of the first drive power source, such that the output torque of the second drive power source is reduced as much as possible. The present arrangement prevents a continued operation of the second drive power source in the first output toque range in which the output torque is comparatively large, so that the second drive power source is kept operable.
In another preferred form of the vehicle control apparatus according to the third aspect of the invention, the second-power-source control means reduces the output torque of the second drive power source across a boundary of two adjacent ones of the plurality of output torque ranges, at a rate lower than a rate at which the second-power-source control means increases the output torque across the boundary. This arrangement prevents a rapid reduction of the drive force of the wheels driven by the second drive power source, assuring running stability of the vehicle. When the selected output torque range of the second drive power source is changed from the range whose maximum drive toque is relatively large to the range whose maximum drive torque is relatively small, the second-power-source control means reduces the output torque of the second drive power source at a rate lower than the rate at which the output torque is increased when the selected output torque range is changed from the range whose maximum drive torque is relatively small to the range whose maximum drive torque is relatively large. Accordingly, an abrupt reduction of the drive force of the wheels driven by the second drive power source is avoided, assuring a sufficiently high degree of running stability of the vehicle.
In a still further preferred form of the vehicle control apparatus according to the third aspect of the invention, the output-torque-range selecting means selects the above-indicated first range when the automotive vehicle is placed in any one of a starting state, a slipping state of its drive wheels and an understeering state, and selects the above-indicated second range when the automotive vehicle is placed in any other states. This arrangement permits the second drive power source to be operated to provide a sufficiently large drive torque when the vehicle is in a starting or understeering state or when the drive wheels are slipping on the road surface. For instance, the output-torque-range selecting means is adapted to select the first output torque range when the wheels driven by the first drive power source are slipping. The present form of the vehicle control apparatus is effective to permit smooth starting and acceleration of the vehicle, and prevent slipping of the wheels driven by the first drive power source and an understeering state of the vehicle.
In a yet further preferred form of the vehicle control apparatus according to the third aspect of this invention, the output-torque-range selecting means selects the above-indicated first range when a gradient of a road surface on which the automotive vehicle lies is larger than a predetermined threshold, and selects the above-indicated second range when the gradient is not larger than the predetermined threshold. This arrangement is effective to prevent or reduce the reverse downward movement of the vehicle on an uphill road surface, while maximizing the frequency of operation of the second drive power source in the second output torque range so as to provide the required minimum drive torque. Thus, the present arrangement assures improved efficiency of operation of the second drive power source and effectively prevents overheating of the second drive power source, particularly where the second drive power source is an electric motor.
In still another preferred form of the vehicle control apparatus according to the third aspect of this invention, the second-drive-source control means operates the second drive power source during starting of the automotive vehicle on an uphill road surface, such that the automotive vehicle is kept driven in a four-wheel driving mode with the pairs of front and rear wheels, until the running speed of the vehicle is raised to a higher value when the gradient of the uphill road surface is relatively large, than when the gradient is relatively small. This arrangement is effective to prevent or reduce the reverse downward movement of the vehicle on the uphill road surface when the vehicle is started on the uphill road surface.
Preferably, the vehicle control apparatus according to the third aspect of the invention further comprises (a) an ant-lock braking-pressure control means operable upon detecting of slipping of any one of the wheels on the basis of the peripheral speed of each wheel detected by a wheel speed sensor, for controlling a braking force to be applied to each slipping wheel such that a slip ratio of the slipping wheel is held within a predetermined range, and (b) vehicle-turning-stability control means operable during turning of the vehicle, for controlling a braking force or drive force of an appropriate one or ones of the wheels so as to prevent an understeering or oversteering state off the vehicle, such that a direction of running of the vehicle does not deviate from a nominal running path of the vehicle defined by a steering angle of a steering wheel of the vehicle, and wherein the second-power-source control means terminates or inhibits an operation of the second drive power source when said wheel speed sensor is defective, or when one of the anti-lock braking-pressure control means and the vehicle-turning-stability control means is in operation. This arrangement automatically changes the vehicle driving mode from the four-wheel driving mode to the two-wheel driving mode (in which the vehicle is driven with only the first drive power source), when the wheel speed sensor is defective or when the anti-lock braking-pressure control means or the vehicle-turning-stability control means is in operation. The present arrangement prevents a control interference between different controls including the control of the second drive power source by the second-power-source control means, the anti-lock braking-pressure control by the anti-lock braking-pressure control means and the vehicle-turning-stability control by the vehicle-turning-stability control means, so that the running stability and safety of the vehicle is improved.
Preferably, the vehicle control apparatus according to the third aspect of the invention further comprises low-temperature detecting means for detecting that an ambient temperature of the vehicle is lower than a predetermined lower limit below which a friction coefficient of a road surface on which the vehicle is running is expected to be lower than a predetermined lower limit, and wherein the second-power-source control means operates the second drive power source when the ambient temperature lower than the lower limit is detected by the low-temperature detecting means. In this arrangement, the second drive power source is automatically activated upon detection of the ambient temperature lower than the lower limit, so that the running safety of the vehicle is improved.
Preferably, the control apparatus according to the third aspect of this invention further comprises (a) vehicle-starting detecting means for determining whether the vehicle is in the process of being started, (b) wheel-slip detecting means for determining whether the wheels are slipping on a road surface, (c) understeering detecting means for determining, on the basis of a steering angle and a yaw rate of the vehicle, whether the vehicle is turning in an understeering state, (d) vehicle-turning detecting means for determining whether the vehicle is turning with the steering angle being larger than a predetermined threshold, (e) accelerator operation detecting means for determining whether a vehicle accelerating member is operated at a rate higher than a predetermined threshold, (f) high-load running detecting means for determining whether the vehicle is running under a relatively high load with the vehicle accelerating member being operated by more than a predetermined amount, and (g) vehicle-deceleration detecting means for determining whether the vehicle is in a decelerating state, and wherein the second-power-source control means determines that the vehicle is required to be driven in the four-wheel driving mode, and operates the second drive power source, when an affirmative decision is obtained by any one of the vehicle-starting detecting means, the wheel-slip detecting means, the understeering detecting means, the vehicle turning-detecting means, the accelerator operation detecting means, the high-load running detecting means and the vehicle-deceleration detecting means, the second-power-source control means determining that the vehicle is not required to be driven in the four-wheel driving mode when a negative decision is obtained by all of the above-indicated seven detecting means, and turning off the second drive power source a predetermined delay time after the moment of the determination that the vehicle is not required to be driven in the four-wheel driving mode. According to this arrangement, the second drive power source is automatically operated when the vehicle driving in the four-wheel driving mode is required, so that the vehicle can be driven with high stability. Further, the delay time provided after the determination that the four-wheel driving of the vehicle is not required and before the second drive power source is turned off is effective to prevent a on-off control hunting of the second drive power source.
Preferably, the vehicle control apparatus according to the above-arrangement further comprises at least one of (a) steering-angle sensor defect detecting means for detecting a defect of a steering angle sensor for detecting the steering angle of the steering wheel of the vehicle, and (b) a yaw-rate sensor defect detecting means for detecting a defect of a yaw rate sensor for detecting the yaw rate of the vehicle, and wherein the second-power-source control means does not operate the second drive power source if the defect of either one of the steering angle sensor and the yaw rate sensor is detected, even when the understeering detecting means has detected the understeering state of the vehicle. This arrangement prevents the vehicle driving in the four-wheel driving mode even if the understeering state of the vehicle is erroneously detected due to a defect of the steering angle sensor or yaw rate sensor.